Lemma 3.3 ([1]}, Theorem 2.1) 
A set \(S\subseteq \mathbb {Z}\)  is a set of recurrence if and only if for all \(\delta >0\) , there is a finite subset \(S_{\delta }\)  of \(S\)  such that \(S_{\delta }\)  is a set of \(\delta \) -recurrence.

